The present invention relates to microbiological processes for the oxidative pretreatment of refractory gold and base metal ores and concentrates, and more particularly, to an apparatus for testing the suitability of such ores for heap biooxidation treatment.
Precious metals are found throughout the world as ores within the Earth""s crust, on the crust surface and dispersed in bodies of water. The precious metal is nearly always in an unrefined state intimately associated with other minerals such as sulfur in the form of arsenopyrite or pyrite. To extract the metal, ore must be processed to remove contaminating minerals such as sulfur, carbon and iron. A commonly used processing technique is cyanidation which involves, quite simply, leaching the ore with cyanide. The cyanide leaches the ore, releasing the precious metal from its association with the gangue minerals. Precious metal is leached into a liquid phase from which it can be recovered.
Gold ores are categorized into two typesxe2x80x94free milling and refractoryxe2x80x94depending on their refractoriness to cyanidation treatment. Free milling ores generally have a low sulfur content and are most often processed by simple gravity techniques or direct cyanidation. Refractory ores, having a higher sulfur content, are difficult to process due to a high content of metallic sulfides, such as pyrite, arsenopyrite and other matter, and require more complex extraction methods. One of the most common of such measures is oxidation.
Oxidation of refractory ores involves a pretreatment step in which the ore is subjected to a roasting or pressure-oxidation technique, typically in conjunction with a pre-concentration process. Increasingly, biooxidation is being used as the pretreatment of choice in substitution for these other more traditional oxidation processes. In biooxidation, the metal sulfides in ore are oxidized in a microbial pretreatment step, prior to the cyanidation step. Specifically, the bacteria oxidize both iron and sulfur under acidic conditions. Oxidation of iron sulfide particles causes the solubilization of iron as ferric ion and sulfide as sulfate ion. This liberates the encapsulated precious metal and makes it amenable to a leaching agent, such as cyanide. The precious metal is subsequently recovered from the oxidized materials by cyanidation, carbon-in-leach or thiosulfate leaching processes.
The adaptation of bacteria in the biooxidation process to recover precious metals from refractory ores has been previously described in a number of variations. For example, one method involves oxidizing multi metallic sulfide ores using a combination of chemical/biological leaching processes and at least three different types of bacteria (U.S. Pat. No. 4,987,081). Bacterial cultures of Thiobacillus thiooxidans, Thiobacillus ferrooxidans and Leptospirillum ferrooxidans are first adapted to high dissolved arsenic concentrations and low pH by subjecting the cultures in a solution containing dissolved arsenic to successive incremental concentrations of arsenic while operating in a continuous mode.
Another process involves the biological oxidation of sulfide in sulfide-containing gold ore followed by cyanide leaching (U.S. Pat. No. 5,006,320). This method involves a further processing step for aerating microorganisms during the oxidation step followed by a subsequent extraction of the metal value from the biooxidized ore.
Biooxidation is not limited to the treatment of gold ores. A related method for producing nickel from sulfide ore involves oxidation by heap leaching (Canadian Patent No. 2,155,050). According to this method, nickel ore, which contains a substantial amount of iron, is subjected to a biological oxidation step and separated from iron into an eluate solution. Nickel is removed from the solution by solvent extraction or by use of an ion exchange resin and subsequent electrowinning of the ferronickel.
Metals can also be recovered from refractory sulfide ores by first separating the crushed ore into a fines and a coarse fraction (U.S. Pat. No. 5,573,575). A heap is formed from the coarse fraction and a concentrate is produced from the fines. The concentrate is then added to the heap for biooxidation. Alternatively, biooxidation of sulfides in the mineral ores may be done by forming particulates. A heap of particulates is formed and a leaching solution is circulated within the heap (U.S. Pat. No. 5,246,486). A variation on this technique involves polymer agglomeration to aid in the removal of particulates from the metal ore (U.S. Pat. No. 5,332,559).
Metals can also be recovered from a refractory sulfide ore by first separating the clays and fines from the crushed ore, and forming a heap from the crushed refractory ore (U.S. Pat. No. 5,431,717). If there is a sufficient amount of precious metal in the separated clays and fines, these materials are further processed. Methods for the biooxidation of refractory carbonaceous or carbonaceous-sulfidic ore material using a specific carbon-deactivating microbial consortium have also been used with varying degrees of success (U.S. Pat. No. 5,244,493).
Preg-robbing by carbon and carbon-containing compounds is also a major problem interfering with efficient recovering of metals from refractory ores. One process to overcome this problem uses leaching with a thiosulfate lixiviant to selectively remove the metal (U.S. Pat. No. 5,354,359). This process involves contacting particulates containing precious metal and preg-robbing carbonaceous components with a thiosulfate lixiviant solution forming stable precious metal thiosulfate complexes. The lixiviant solution is recovered after it has had time to become loaded with the metal in the ore material.
Leaching has also been used to remove copper from copper sulfide-containing ore (U.S. Pat. No. 4,571,387). According to this process, ore is ground and mixed with an aqueous acid-leaching medium containing sulfide-oxidizing bacteria, a bacterial nutrient and a catalytic amount of silver. Carbon dioxide and oxygen are provided as well as a bacterial compatible acid. The basic leaching process has been enhanced to increase the leaching rate of a mineral when the mineral is characterized by the tendency to form a reaction product layer during leaching (U.S. Pat. No. 4,343,773). A particulate modifier such as carbon is mixed with the mineral before leaching and selectively alters the characteristics of the reaction product layer.
Prior to incurring the substantial costs inherent in scaling up to biooxidize a particular ore, the ore under consideration typically is batch tested to determine if it is suitable for biooxidation. However, conventional testing procedures can take as long as six months to complete due to the time needed for adaptation of the bacteria and the lag phase between inoculation and the onset of oxidation.
The present invention overcomes the problems and disadvantages associated with current strategies and designs and provides a novel column reactor for use in the processing of refractory ores.
One embodiment of the invention is directed to a novel column reactor which simulates continuous operating conditions of heap biooxidation and other types of static reactors. The novel reactor of the present invention can be used to facilitate the testing of refractory gold and base metal sulfide ores for their suitability to heap biooxidation or bio-leaching. Such base metal sulfide ores include base and precious metals. The reactor contains features of heap biooxidation processes such as integral inoculum generation, and can be used to generate process engineering and design data for such processes. The reactor of the present invention incorporates novel means for the simultaneous aeration of the charge, collection of solution samples and regeneration of bacteria and overcomes one of the fundamental problems of biooxidation systems, namely, that of instability.
The present invention incorporates a novel inoculum regeneration system comprising both an air-sparged reactor and electrocell. This combination of reactors overcomes the oxygen mass transfer limitations of the electrocell such that the only limitation becomes cell size.
Accordingly, one embodiment of the invention is directed to a biooxidation apparatus for simulating heap biooxidation comprising: a combination plenum chamber/bacterial regenerator coupled to the oxidation chamber, the regenerator comprising means for providing oxygen to a population of bacteria being regenerated and means for providing a ferrous iron substrate to the population of bacteria; means for recirculating the population of bacteria from the bacterial regenerator through the electrocell; and means for aerating the plenum chamber.
Still another embodiment is directed to a biooxidation apparatus for evaluating the suitability of a refractory ore to heap biooxidation. Another embodiment of the invention is directed to an apparatus for generating a selected population of bacteria useful for the biooxidation of a specific refractory ore.
Still another embodiment is directed to a method for generating a selected population of bacteria useful for the biooxidation of a specific refractory ore. Another embodiment is directed to a novel method for evaluating the suitability of a refractory ore for biooxidation. Still other embodiments of the invention are directed to novel methods and apparatuses for providing a continuous supply of ferrous iron and oxygen to biooxidative bacteria to enhance growth, and to methods and apparatuses for recycling effluent from the biooxidation of refractory ores.
Other embodiments and advantages of the invention are set forth, in part, in the description which follows, and, in part, will be obvious from this description and may be learned from the practice of the invention.